1. FIELD OF THE INVENTION
The present invention concerns a magneto-optical disk for recording, reproducing and erasing information, for example, by laser beams.
2. DISCUSSION OF BACKGROUND
With the remarkable development of so-called information society in recent years, requirements for recording media of large capacity and relevant recording/reproducing system have been increased rapidly and laser video disks, digital audio-disks, write-once disks for use in image or document files have already been put to practical use. However, such recording media put to practical use at present have a drawback of incapable of rewriting, and rewritable recording media and recording/reproducing system have been demanded. Then, as one of recording/reproducing systems, magneto-optical recording/reproducing systems have been proposed and shipping for their samples has already been started.
Magneto-optical disks used in the magneto-optical recording/reproducing system basically comprise a plastic substrate or a glass substrate on which an amorphous vertical magnetization memberane (or, a thin film of amorphous magnetic material having magnetic anisotropy so that the magnetic material is magnetized with its magnetic axis perpendicular to the plane of the thin film) is formed, and rare earth-transition metal alloys such as GdTbFe, TbFeCo, DyFeCo, GdDyFe, etc. are used as the magnetic material.
The written signal in a magneto-optical disk is read out by utilizing Kerr effect, that is, the rotation of the direction of polarization on reflection. The carrier-to-noise ratio (CNR) of read signal is generally insufficient for practical use because the Kerr rotational angle is small. In view of the above, it has been attempted to increase the Kerr rotational angle by forming a dielectric film of SiO or SiO.sub.2 on the magnetic material.
However, since the rare earth-transition metal alloy film is not superior in anticorrosion property, it is required that the dielectric film is effective not only for increasing the Kerr rotational angle but also for protecting the corrosion of magnetic film. The SiO or SiO.sub.2 film has a drawback of incapable of providing substantial protection against corrosion. As for the improvement of the drawback, Si.sub.3 N.sub.4 or A1N has been proposed as the dielectric film. As the example for the former, Japanese Patent Publication No. Sho 62-27458 discloses the use of a silicon nitride film, which increases the Kerr rotational angle and which is suitable to practical use.
When the silicon nitride film is deposited by means of sputtering, since the film having such a refractive index as that increases the Kerr rotational angle sufficiently involves large internal stresses therein, the film has often been peeled off after the preparation of the disk and lacks in the reliability. Furthermore, since the silicon nitride films deposited on the inner surface of the sputtering system peel off owing to the large internal stresses, there is a problem that dusts increase in the sputtering system and result in high defect density of the disk.